Tumor cell-induced platelet aggregation (TCIPA) is not a new concept, which can be traced back to the late nineteenth century. For pancreatic cancer cells, it was evidenced by six human cell lines, which can induce platelet aggregation via activation of thrombin (Heinmöller et al., 1995). Several molecular pathways are involved in pancreatic TCIPA. Activated phospholipase A2 enzymes release arachidonic acid (AA) which is a precursor of thromboxane A2 (TXA2). Cyclooxygenase 1 (COX-1) catalyzes the transformation of AA into TAX2, which is important for platelet aggregation. TAX2 can activate the thromboxane receptor-induced changes of platelet shape, activation of integrins, and degranulation. Moreover, the expression of COX-2 is reported as increased in tissues of pancreatic cancer (Ding et al., 2003; Sangkuhl et al., 2011). P-selectin and tissue factor (TF) accumulation are associated with platelet activation and platelet-rich thrombus (Wang et al., 2012; Mezouar et al., 2015; Hisada et al., 2017). The increased von Willebrand Factor (VWF), a large polymeric glycoprotein, is involved in the adhesion and aggregation of platelets. Cancer patients are associated with a higher risk of venous thrombosis. Especially, high VWF levels were observed in pancreatic, lung, brain, stomach, and colorectal cancer patients (Obermeier et al., 2019). Podoplanin on the surface of cancer cells induces platelet aggregation. Pancreatic cancer-associated fibroblasts (CAF) reportedly express podoplanin (Takemoto et al., 2017; Suzuki-Inoue, 2019). In addition, a recent study showed pancreatic cancer (Paca) cells can stimulate the rapid release of neutrophil extracellular traps (NETs) and promote thrombus formation (Abdol Razak et al., 2017).

In patients diagnosed with pancreatic cancer at the head of the pancreas, platelet count and concentration of vascular endothelial growth factor (VEGF) released from platelets were significantly increased (Sabrkhany et al., 2017). Mean platelet volume (MPV) is changed in different stages of pancreatic cancer (PC). MPV was elevated in PC patients with synchronous liver metastases and stage III-IV (Yin et al., 2018). However, MPV was decreased in resectable PC patients with poor prognoses (Yagyu et al., 2021). Best et al. (2015) evidenced mRNA profiles of tumor-educated blood platelets (TEPs) were different between KRAS mutant pancreatic cancer and KARS wild-type. TEPs RSL24D1 mRNA was negatively related to early pancreatic cancer compared to healthy controls (Xue et al., 2018). Moreover, the platelet proteome of patients with head of pancreas cancer (stage I-II) is significantly different from that of healthy individuals of equivalent sex and age (Sabrkhany et al., 2017, 2018).

Extracellular vesicles (EVs) are a means that facilitate the exchange of a broad array of molecules between adjacent or distant cells. Platelet EV cargo includes lipids, protein, nucleic acids, and organelles, which can enter lymph, bone marrow, and synovial fluid. EVs are classified into exosomes (30–150 nm), microvesicles—also referred as microparticles or ectosomes—(100–1,000 nm), and apoptotic bodies (1,000–3000 nm) (Ferreira et al., 2020). Platelet-derived extracellular vesicles (PDEVs) are the most abundant type of EVs in the circulation. Microparticle (MP) refers to particles released from the surface of cells, especially in the field of platelet research (Supplementary Table 2). Patients with pancreatic cancer had significantly increased levels of MP-associated TF activity compare with healthy controls (Tesselaar et al., 2007; Tilley et al., 2008). MP-TF activity had a strong association with mortality in pancreatic cancer, which could be a marker for aggressive cancer phenotype (Thaler et al., 2012). In addition, circulating microvesicles (MVs)-associated thrombin generation is different between patients and healthy control (Hellum et al., 2017).

Platelet messenger RNA (mRNA) profile is currently emerging as a new potential biomarker in cancer diagnosis. A study demonstrated platelets isolated from glioma or prostate cancer patients contained the cancer-associated mRNA transfer EGFRvIII and PCA3 (Nilsson et al., 2011). Recent studies highlighted pancreatic tumors can alter platelet RNA profiles (Best et al., 2015). Platelets from cancer patients contained tumor-associated RNA biomarkers, indeed, mRNA sequencing of tumor-educated platelets can identify pancreatic cancer patients with 96% accuracy.

MPV is associated with the overall survival of pancreatic cancer patients (Yin et al., 2018). Particularly, large platelet size is associated with poor outcomes in patients with metastatic pancreatic cancer (Lembeck et al., 2019). However, reduced MPV levels predict shorter survival in patients after surgery (Yin et al., 2020). There is a novel scoring system based on hemostatic parameters that showed platelet count was an independent prognostic factor in advanced pancreatic cancer (Zhang et al., 2019). In many studies, CA19-9 decrease during treatment has been related to longer survival of pancreatic cancer. Moreover, the correlation of CA19-9 decreases; overall survival was stronger in advanced pancreatic cancer with fewer platelets (Chen et al., 2019). The preoperative platelet to lymphocyte ratio (PLR) was reported to be a significant independent prognostic factor in patients (Song et al., 2017). In addition, the increased PLR is also related to a poor long-term prognosis in resected pancreatic cancer (Yu et al., 2018; Negoi et al., 2019). Combining PLR and CA19-9 values could allow earlier diagnosis of pancreatic cancer patients with type 2 diabetic patients (Qin et al., 2019). Preoperative platelet-to-albumin ratio (PAR) was reported as a novel significant independent prognostic index for disease-free survival (DFS) and overall survival (OS) in patients after pancreatic resection (Shirai et al., 2017). Moreover, platelet receptors and platelet-derived factors are also involved in cancer progression (Supplementary Table 1). The platelet-derived growth factor (PDGF) signaling pathway plays an important role in the progression of pancreatic cancer. Duan et al. (2018) analyzed three published genome-wide association study datasets to observe genetic variants in the PDGF subunit B gene associated with pancreatic cancer risk in European populations. High levels of circulating PDGF-AA serve as a predictor of poor cancer-specific survival, whereas high levels of PDGF-BB are associated with a favorable prognosis (Rahbari et al., 2011; Kahlert et al., 2014). In addition, PDGFR beta (PDGFRβ) is more frequently expressed in primary endocrine pancreatic tumors (EPTs) and metastases as compared to normal endocrine pancreatic tissue (Fjällskog et al., 2007). PDGFRβ is a marker of activated pancreatic stellate cells (PSCs) which play a vital role in desmoplasia. Higher expression of PDGFRβ matched shorter prognosis as well as lymphatic invasion and lymph node metastasis (Yuzawa et al., 2012).

Recent researches evidence that platelets have a direct effect on tumor cell proliferation. PANC-1 cancer cell proliferation was potentiated by human platelets in a manner dependent on the upregulation and activation of the oncoprotein c-MYC (Mitrugno et al., 2017). Platelet-derived growth factor (PDGF) is an important cytokine in pro-proliferative and invasion signaling, which plays a key role in the regulation of interactions between pancreatic cancer cells and adjacent stroma (Haqq et al., 2016). Moreover, dual-specificity phosphatase 28 (DUSP28) regulates chemo-resistance and migration in pancreatic cancers. PDGF-AA was evidenced in a public microarray database and in vitro assay, which is a critical role in pancreatic cancer malignancy. In addition, DUSP28 and PDGF-AA formed an acquired autonomous autocrine-signaling pathway. Targeting DUSP28 inhibited the tumor growth and migratory features through the blockade of PDGF-AA expression and intracellular signaling (Lee et al., 2017). Mucin 1 (MUC1), a transmembrane mucin glycoprotein, regulates PDGFA expression and secretion in pancreatic cancer cells, accordingly, influences the proliferation of pancreatic tumors (Sahraei et al., 2012). There are some treatments for targeting PDGF to decrease the proliferation of pancreatic cancer cells (Inoue et al., 2017; Salem et al., 2020). PDGF-BB mediates pancreatic cancer growth via regulation of the Hippo/Yes-associated protein signaling pathway (Li et al., 2021). NF-κB is the downstream stage of the AA pathway. NF-κB and activator protein 1(AP-1) can bind to COX-2, lipoxygenases (LOXs), and phospholipase A2 (PLA2), which play a pro-tumorigenic role in pancreatic cancer (Gong et al., 2014; Matejcic et al., 2018). COX-2 is only expressed in pancreatic islets and has no expression in normal exocrine pancreatic tissues. Numerous clinical studies reported that mRNA and protein expression of COX-2 are up-regulated in pancreatic cancer. The use of aspirin, the major pharmacological inhibitor of COXs, was negatively related to the incidence risk of pancreatic cancer (Sun et al., 2019). Moreover, low-dose aspirin (81–162 mg orally daily) is relatively selective for COX-1 inhibition, interaction with circling tumor cells, and platelet aggregation. However, two large cohort studies reported regular aspirin or non-aspirin NSAID use was not associated with future risk of pancreatic cancer. There is only a possible reduction in patients of pancreatic cancer with diabetes (Khalaf et al., 2018). Pancreatic cancer cells express the purinergic receptor P2Y12, that is an ADP receptor found mainly on platelets. Ticagrelor, a P2Y12 inhibitor, decreases the survival signals initiated in cancer cells by platelet-derived ADP and ATP (Elaskalani et al., 2017). Arachidonate 12-lipoxygenase (ALOX12) and 12-hydroxyeicosatetraenoic acid contribute to stromal aging-induced progression of pancreatic cancer (Sarsour et al., 2020). 5-lipoxygenases (5-LOX) were found overexpressed in the tissues of pancreatic cancer, the inhibition of 5-LOX induces apoptosis in pancreatic cancer cells (Zhou et al., 2015). In addition, P-selectin deficiency and soluble P-selectin abolish platelet deposition within tumors, decreasing the secretion of vascular endothelial growth factor and angiogenesis, thereby suppressing tumor growth (Qi et al., 2015). Rivipansel inhibits selectins and decreases the recruitment of plasma cells in multiple myeloma (Azab et al., 2012). Crizanlizumab, a selective blocking antibody of P-selectin, also is indicated as a potential treatment option for patients with pancreatic cancer in the future. Selected clinical trials of glycobiology-targeted therapeutics for pancreatic cancer are in Phase I, for example MVT-5873 and MVT-10775 (Smith and Bertozzi, 2021). The first study for platelets involved in tumor angiogenesis is by Pinedo et al. (1998) They proposed platelets as a rich source of stimulators and inhibitors of angiogenesis and their interaction with the endothelium. Starlinger et al. (2011) compared different methods to get circulating platelet-stored angiogenesis factors in pancreatic cancer patients, and a significant increase of thrombospondin 1(TSP-1) and platelet factor 4 (PF-4) were determined. The overexpression of VEGF and platelet-derived endothelial cell growth factor (PD-ECGF) protein significantly correlated with high microvessel density (MVD) in patient tissues of pancreatic cancer (Fujimoto et al., 1998). VEGF is a chemotactic vascular permeability factor stored in α-granules and released from activated platelets. The study evidenced that VEGF expression correlated significantly with increased intratumoral microvessel density (IMD), which are important regulators of pancreatic tumor angiogenesis and predictive of benefit from adjuvant therapy (Khorana et al., 2005). Platelet factor 4 (PF-4) inhibits angiogenesis in vivo and in vitro (Maione et al., 1990). PF-4 modulates fibroblast grow factor 2 (FGF-2) activity (Perollet et al., 1998). However, a study reported that FGF-1 and FGF-2 treatment led to the induction of phosphorylation of E-cadherin and β-catenin on tyrosine residues, resulting in angiogenesis in pancreatic cancer cells (El-Hariry et al., 2001). Accordingly, FGFR also plays a key role in tumor angiogenesis, downregulations of FGFR-2 led to decreased phosphorylation of ERK and VEGF-A in PDAC cells after FGF-2 stimulation (Compagni et al., 2000; Kang et al., 2019). Platelets are the sole source of EGF in circulation, however, inhibition of EGFR tyrosine kinase activity suppresses pancreatic tumor angiogenesis (Bruns et al., 2000). Interleukin-8 (IL-8) is a chemokine related to PF-4, which activates G-protein coupled receptors as a pro-angiogenic factor (Le et al., 2000). Platelets contain 40–100 times more TGF-β than other non-neoplastic cells (Assoian et al., 1983). Although TGF-β is a pro-angiogenic factor in many cancers, the role in PDAC is controversial. TGF-β interferes with a soluble TβRII, which suppresses pancreatic cancer angiogenesis (Rowland-Goldsmith et al., 2002). Contrarily, other studies evidenced TGFβ1 is overexpressed in PDAC, and it induces plasminogen activator inhibitor-1(PAI-1) expression in pancreatic cancer cells, promoting angiogenesis in vivo (Andreasen et al., 2000). TGF-β can promote stromal activation, which induces angiogenesis and attenuates a productive anti-tumor immune reaction (Hinz et al., 2007; Kano et al., 2007). Similarly, because pancreatic cancer has a highly hypoxic microenvironment, in which hypoxia-inducible factor-1α (HIF-1α) is activated. HIF-dependent pathways subsequently activate HGF/c-MET signaling pathway in pancreatic tumor cells, and induce angiogenesis (Kitajima et al., 2008). HDF can also be anti-angiogenic due to alternative splicing of the α-chain (Date et al., 1998). Platelets can induce several matrix metalloproteinase (MMPs) in platelet-tumor cell interactions. Integrin engagement leads to the secretion of MMP-2, MMP-9 and surface expression of MT1-MMP. MMP-9 is essential to angiogenesis in pancreatic cancer mice model (Nakamura et al., 2007). Expression of MMP-2 and MMP-9 mRNA are associated with microvessel density in pancreatic cancer patients (Xiang et al., 2017). Platelet-derived microparticles (PMP) can enhance tumor growth by the release of potent growth factors in the tumor micro-environment (Goubran et al., 2015).

That platelets promote tumor metastasis is not a new idea. Activation of platelets and the TF-thrombin-PAR-1 pathway are reported to promote metastasis of PDAC cells (Yang et al., 2019). The activation of platelets increases lysophosphatidic acid (LPA) release from platelets, LPA in turn enhanced tumor cell invasiveness and cell migration (Yoshikawa et al., 2013). In addition, vWF-platelet interactions also promote a number of metastases (Patmore et al., 2020). Tumor cell-derived MMPs can elicit platelet activation, accordingly, platelet-activating factors can induce an increased MMP expression, and inhibition of MMP reduces both growth of pancreatic cancer metastases and the death rate (Jimenez et al., 2000). High TGF-β induced expression in PDAC patients is associated with pancreatic cancer cell migration (Costanza et al., 2019). Moreover, TGF-β has a role as a suppressor of stromal promotion or through alterations in pancreatic stellate cell MMP profiles with subsequent inhibition of pancreatic cancer cell migration (Tjomsland et al., 2016). Platelets secrete a mountain of growth factors and chemokines, such as VEGF, PDGF, CXCL5, to help establish metastatic foci, neovessel formation, and metastasis. In addition, inhibition of platelets activation prevents the P-selectin and integrin-dependent accumulation of pancreatic cancer cell microparticles and reduces metastasis (Mezouar et al., 2015). ADAM9 contributes to vascular invasion and is involved in metastasis (Oria et al., 2019). Epithelial-mesenchymal transition (EMT), a transient and reversible process, promotes cell motility, invasion, and dissemination of cancer cells out of the tumor microenvironment. Extravasated platelet aggregation is associated with the first step in the formation of EMT. Primary tumor cells surrounded by platelets exhibited characteristics of EMT in pancreatic cancer (Miyashita et al., 2015).

A few studies have shown platelets can influence the efficacy of chemotherapy. Platelets increase the resistance of colon and ovarian cancer to 5-fluorouracil and paclitaxel (Radziwon-Balicka et al., 2012). Platelet factors hinder the cytotoxicity of sorafenib and regorafenib in hepatocellular carcinoma by increasing the phosphorylation of ERK and p38 (D’Alessandro et al., 2014). Elaskalani et al. (2017) evidenced that platelet-derived ADP and ATP promote pancreatic cancer cell survival and gemcitabine resistance. Ticagrelor, an inhibitor of ADP-P2Y12 axis, enhances chemotherapeutic efficacy in pancreatic cancer cells by targeting the Novel P2Y12-AKT pathway (Elaskalani et al., 2020). In addition, platelets also drive EMT to promote chemotherapy resistance. β1 integrin-dependent interaction within the tumor microenvironment may alter tumor response to chemotherapy.